Broadband communications are emerging for wireline and wireless communications, such as DVB-H (470-890 MHz), cognitive radios (0.1-10 GHz), etc. A low-noise amplifier (LNA) circuit may be placed at the first stage of an entire receiver. Thus, the properties of low-noise amplifier circuits directly impact the performance of the receiver. Current challenges arising in handling wide-spectrum capacities include designing LNAs with wideband input match and gain bandwidth. However, conventional approaches to achieve these performance requirements incur a poor noise figure (NF).
The desired frequency span can be processed by one LNA circuit or by several LNA circuits each for processing a smaller band. A single-ended LNA is efficient because antennas and RF filters usually produce single ended signals, and conserves area, power, and costs. On the other hand, differential signaling in the receive chain reduces second-order distortion and rejects power supply and substrate noise. At some point in the receive chain, a balun may be used to convert the single-ended RF signal into a differential signal.
By way of example, a single-input differential-output low-noise on-chip amplifier, i.e., a balun LNA, facilitates connection to the front antenna and to the following mixer of a double balanced topology. A balun is a type of electrical transformer that converts electrical differential signals balanced about ground to unbalanced (single-ended) signals and vice versa. A low-noise amplifier (LNA) is an electronic amplifier used to amplify very weak signals (for example, captured by an antenna). FIG. 1 is a block diagram depicting an example of a single-stage balun LNA composed by common-gate (CG) common-source (CS) amplifiers. The balun LNA eliminates the need of an off-chip balun in front of the LNA, and provides a low noise feature and low external BOMs (bill-of-materials). In addition, the balun LNA does not require an on-chip balun after the LNA, such that it is effective for low distortion as well as low power consumption. FIG. 2 is a simplified schematic of the CG-CS balun amplifier to show the noise transfer functions of FIG. 1. A common-gate amplifier is typically used as a current amplifier, while a common-source amplifier is typically used as a voltage or transconductance amplifier.
The common-gate (CG) amplifier as shown in FIG. 2 is applied for its wideband impedance match. The problems of a noise figure (NF) greater than 3.5 dB and an insufficient voltage gain are alleviated by the parallel common-source (CS) amplifier. When the CG and CS amplifiers are equally sized, the noise figure of the CG-CS balun amplifier is still too high.
Noise figure (NF) is a measure of degradation of the signal-to-noise ratio (SNR), caused by components in a radio frequency (RF) signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard noise temperature T0 (usually 290 K). The noise/distortion from MCG (M1) is canceled by taking the output differentially. The MCS (M2) noise dominates the NF. Thereafter, the parallel CS amplifier doubles the voltage gain and cancels the noise and distortions generated from the CG amplifier. The CG-CS input stage also naturally performs single-to-differential conversion, which may be provided a low-cost implementation without an extra off-chip balun.
Referring to FIG. 2, assuming the CG and CS branches are symmetrical, i.e., gm,CG=gm,CS and RL1=RL2=RL, the overall noise figure of the entire balun LNA can be approximated by:
                    F        =                  1          +                                                                      g                                      m                    ,                    CG                                                  ⁢                                  R                  s                                                                                              T                                      i                    ,                                          R                      s                                                                                                            ⁢                          {                                                                    (                                          γ                      α                                        )                                    ·                                      (                                                                                                                                                  T                                                          i                              ,                              CG                                                                                                                                2                                            +                                                                                                                              T                                                          i                              ,                              CS                                                                                                                                2                                                              )                                                  +                                  2                                                            g                                              m                        ,                        CG                                                              ⁢                                          R                      L                                                                                  }                                                          (        1        )            
Where gm,CG and gm,CS respectively represent the transconductance of the CG and CS amplifiers. Transconductance (gm), also known as mutual conductance, is the ratio of the current change at the output port to the voltage change at the input port. RL1 and RL2 respectively represent the resistor loads of the CG and CS branches. Rs is the source resistance and equal to 50 ohm. Ti,CG and Ti,CS respectively denote the current transfer gains in the differential outputs due to the CG and CS transistor noise currents, i.e., in,M1 and in,M2 are respectively shown as follows:
                                          i                          n              ,                              M                ⁢                                                                  ⁢                1                                              =                      4            ⁢                                          kT                ⁡                                  (                                      γ                    α                                    )                                            ·                              g                                  m                  ⁢                                                                          ⁢                  1                                                                    ,                              i                          n              ,                              M                ⁢                                                                  ⁢                2                                              =                      4            ⁢                                          kT                ⁡                                  (                                      γ                    α                                    )                                            ·                              g                                  m                  ⁢                                                                          ⁢                  2                                                                                        (        2        )            
Where γ/α represents the transistor's excess noise factor, and Ti,Rs represents the transfer gain in the differential output due to the Rs noise current, i.e., in,Rs.
                              i                      n            ,            Rs                          =                              4            ⁢                                                  ⁢            kT                                R            s                                              (        3        )            
CG transistor channel noise in,CG results in the noise currents ind1 and ind2 at the drain ports of the CG and CS amplifiers, respectively, and Ti,CG is the difference of these two components normalized to in,CG. These parameters can be derived as:
                              i                      nd            ⁢                                                  ⁢            1                          =                              1                          1              +                                                g                                      m                    ,                    CG                                                  ⁢                                  R                  s                                                              ·                      i                          n              ,              CG                                                          (        4        )                                          i                      nd            ⁢                                                  ⁢            2                          =                                                            g                                  m                  ⁢                                                                          ⁢                  2                                            ·                              i                                  nd                  ⁢                                                                          ⁢                  1                                                      ⁢                          R              s                                =                                                                      g                                      m                    ,                    CS                                                  ⁢                                  R                  s                                                            1                +                                                      g                                          m                      ,                      CG                                                        ⁢                                      R                    s                                                                        ·                          i                              n                ,                CG                                                                        (        5        )            
It is noted that ind1 and ind2 are of the same sign. Assuming two branches are symmetrical, i.e., gm,CG=gm,CS and RL1=RL2, and the noise current gain ind1/in,CG=k, the other current gain ind2/in,CG=(1−k). On the other hand, the noise current gain resulting from the CS transistor channel noise (in,CS) at the drain ports of CG and CS is 0 and 1, respectively.
The balun LNA provides a voltage gain of 2RL/Rs as 1/gm,CG=Rs. This means the noise from the CG amplifier can be canceled due to the parallel CS amplifier since two output noise are equal, i.e., k=(1−k)=0.5.
Although the noise from the CG amplifier can be canceled due to the parallel CS amplifier, the overall NF is still relatively high due to the significant contribution from the CS amplifier. Based on the equation (1), the NF is as large as 3.5 dB with RL=400 and γ/α=1.
To minimize the noise figure in the equation (1), |Ti,CG|2+|Ti,CS|2 should be minimized. Ti,CG denotes the current transfer gain in the differential output due to the CG transistor noise current, i.e., in,M1, and can be expressed by:
                              T                      i            ,            CG                          =                              1                          1              +                                                g                                      m                    ,                    CG                                                  ⁢                                  R                  s                                                              -                                                    g                                  m                  ,                  CS                                            ⁢                              R                s                                                    1              +                                                g                                      m                    ,                    CG                                                  ⁢                                  R                  s                                                                                        (        6        )            
Assuming 1/(1+gm,CSRs) is k, then the second term gm,CSRs/(1+hm,CGRs) can be expressed as (1−k) since gm,CG=gm,CS. On the other hand, Ti,CS denotes the current transfer gain in the differential output due to the CS transistor noise current, i.e., in,M2, and is equal to one.Ti,CG=k−(1−k)=2k−1=0, Ti,CS=1  (7)
Thus, the term, |Ti,CG|2+|Ti,CS|2 can be rewritten as|Ti,CG|2+|Ti,CS|2=|k−(1−k)|2+|1|  (8)
To minimize the term, |Ti,CG|2+|Ti,CS|2, k=0.5. Therefore, the term, |Ti,CG|2+|Ti,CS|2, has a minimal/optimized value of one, and F=2.25 (−3.5 dB). This is what is shown in the conventional balun LNA circuit topology as depicted in FIG. 1 using CG and CS amplifiers with identical size and bias.
To improve the balun LNA of FIG. 1, a conventional approach suggested an inductorless LNA and derives conditions for simultaneous output balancing, noise canceling and distortion canceling, by admittance scaling the CS-stage with respect to the CG-stage for noise canceling. This approach obtains a noise figure in the order of 3 dB or lower. Another conventional approach suggested a feedforward thermal noise-canceling technique for a LNA, which allows for simultaneous noise and impedance matching, while canceling the noise and distortion contributions of the matching device. This approach provides wide-band impedance-matching amplifiers with a NF range of 1.9-2.4 dB, without suffering from instability issues. This is done by placing an auxiliary voltage-sensing amplifier (with a gain −Av) fed-forward to the matching stage such that the noise from the matching device cancels at the output, while adding signal contributions. In this way, the LNA noise figure is minimized at the price of power dissipation in an auxiliary amplifier. The difference in sign for noise and for signal cancels the noise of the matching device, while simultaneously adding the signal contributions. In other words, the approach creates a new output at a node added to a scaled negative replica of the voltage at another node.
However, these conventional approaches come at a cost of high power consumption (e.g., 14-35 mW) and low average gains (e.g., 14-15.6). In addition to noise figure well below 3 dB, adequate linearity, and source impedance matching, high-sensitivity integrated receivers require LNAs with sufficiently large gain to handle broadband communications while allowing some variable gains to handle interference generated by strong adjacent channels. Other approaches were proposed to overcome power and gain issues.
A third conventional approach suggested a differential common-gate LNA consisting of cross-coupled capacitors and PMOS transistors in a differential CG configuration. The differential common-gate LNA reduces the power consumption to 3.6 mW, yet providing only an average 3 dB NF in the frequency range of 0.3-0.92 GHz.
A fourth conventional approach suggested a LNA with single-ended input and output employing noise and IM2 distortion cancellation for a digital terrestrial and cable TV tuner. The single-ended LNA adopts thermal noise canceling based on current amplification, to acquire low NF (e.g., 3.0-3.3 dB) and high third-order intercept point (IIP3) performance (e.g., 3 dBm) without degradation of gain and wideband input matching. In telecommunications, a third-order intercept point (IIP3 or TOI) is used to measure weakly nonlinear systems and devices (e.g., receivers, linear amplifiers and mixers, etc.). It is assumed that the device nonlinearity can be modeled using a low-order polynomial. The third-order intercept point relates nonlinear products caused by the third-order nonlinear term to the linearly amplified signal.